Usually, powerline modems adapt communication parameters (adaptive modulation) to channel characteristics in order to increase the throughput and reliability. This adaptation requires channel state information at the transmitter and therefore requires feedback from the receiver to the transmitter.
In some cases the transmitter cannot use channel state information, for instance for initial communication (before adapting to the channel characteristics) or for broadcast messages (messages to all stations in the network).
In those cases, powerline modems generally use a robust communication mode (ROBO mode). For instance, the HomePlug standard specifies a ROBO mode where each OFDM (orthogonal frequency-division multiplexing) subcarrier is modulated with the same QAM (quadrature amplitude modulation) constellation (QPSK (quadrature phase-shift keying) in HomePlug). In addition to the forward error correction (FEC), a repetition code is used where each coded bit is repeated two or four times. With these parameters the communication becomes very robust and is designed to work for practically all links within powerline networks.
In order to comply with electromagnetic interference (EMI) regulations some frequencies have to be notched. The frequencies to be notched may vary in different countries. The frequencies to be notched are specified by a so-called tone mask, which is stored in the powerline modems. In order to guarantee interoperability the tone mask has to be the same for all modems (default broadcast tone mask in the HomePlug standard).
The tone masks might be different for different countries. In general, the tone mask for North America (NA mask) is considered the default broadcast tone mask. In a tone mask it is determined which carriers are used during the transmission of data. The tone mask is known at a transmitting side and at a receiving side. Further notched frequencies according to tone masks of other countries can be implemented by using amplitude maps. In the amplitude maps it is specified that no power is allocated to the further frequencies which are additionally to be notched.
If tone masks for different countries do not correspond to each other, performance degradation at the receiver may occur. For example, the receiver expects information on notched subcarriers. To some extent the lost information can be compensated by the forward error correction and copy coding. Copy coding (also called diversity copying in the HomePlug standard) uses frequency diversity (some subcarriers are highly attenuated; some subcarriers show only a small attenuation). However, the copy coding does not take into account the potentially notched subcarriers. Consequently, all copies may fall on the notched subcarriers.
It is an object of the disclosure to provide a transmitter, a communications system and a method for transmitting data which provide enhanced certainty that transmitted data is actually received by a receiver.
The object is achieved by the subject-matter of the independent claims. Further embodiments are specified in the dependent claims, respectively. Details of the disclosure will become more apparent from the following description of embodiments in connection with the accompanying drawings, wherein features of the various embodiments may be combined unless they exclude each other.